1. Technical Field
The present invention relates to a control device that controls an electric waste gate of a turbocharger provided in an engine and, in particular, to a control device capable of appropriately controlling an opening degree of a waste gate valve from a time immediately after a start of the engine.
2. Related Art
A turbocharger that is provided in an engine mounted on an automobile or the like has: a turbine that is driven by exhaust; and a compressor that is driven by the turbine to compress fresh air (i.e. combustion air).
In general, in order to prevent an excess boost of supercharging pressure, a turbo system is provided with: a waste gate passage that bypasses a portion between an upstream and a downstream of the turbine; and a waste gate valve (i.e. WGV) that opens/closes the waste gate passage. When the supercharging pressure is boosted, the turbo system opens the waste gate valve to reduce exhaust gas flowing to the turbine side, reduces a workload of the turbine, and thereby suppresses the supercharging pressure.
In many of the conventional cases, such a waste gate valve is a mechanical valve that is opened when the supercharging pressure reaches predetermined positive pressure. However, in recent years, use of an electric waste gate valve that is opened/closed by an electric actuator with superior conformability has been proposed.
In the case where the electric waste gate valve that is opened/closed by the electric actuator is used, an opening degree thereof can be set to any opening degree including an intermediate opening degree regardless of the supercharging pressure of the waste gate valve.
For instance, immediately after a cold start of the engine, namely, in a state where warming of the engine has not been completed, the waste gate valve is opened at a predetermined opening degree. Some of the exhaust gas bypass and is introduced into a catalyst that is provided on a downstream of the turbine, while a high temperature thereof is maintained. In this way, warming up the catalyst is enhanced, and exhaust gas processing performance can be improved.
When opening control of the electric waste gate valve is executed, the waste gate valve is directly exposed to the exhaust gas and thus reaches the high temperature. Due to thermal expansion of each member thereof, a fully closed position of the waste gate valve is changed.
For this reason, unless the fully closed position that serves as a reference point of the opening control is appropriately learned, the opening degree cannot accurately be controlled.
As related art on fully closed position learning and the like of the electric waste gate valve, for instance, a learning unit that performs first learning and second learning and computing a difference between a learning value acquired in the first learning and a learning value acquired in the second learning is described in Japanese Unexamined Patent Application Publication (JP-A) No. 2015-025409 (Patent Document 1). In the first learning, the waste gate valve is fully closed before the start of the engine. Then, an initial fully closed position of the waste gate valve is learned from a position detected by a detector. In the second learning, the waste gate valve is fully closed after the start of the engine. Then, a zero point of the waste gate valve is learned from a position detected by the detector.
In addition, a reference position learning device is described in JP-A No. 2015-132204 (Patent Document 2). In the reference position learning device, a reference position of a valve that adjusts a flow of exhaust or intake air of an internal combustion engine is learned by a first reference position learning unit at a start of the internal combustion engine, and is learned by a second reference position learning unit when a predetermined learning condition is established after the start of the internal combustion engine. The reference position is updated on the basis of these learning reference positions, and the valve is controlled by using the updated reference position.
In the above-described related art, it is required to drive the waste gate valve once to a fully closed state during the start of the engine, so as to perform the fully closed position learning. Thus, while the fully closed position learning is performed, the exhaust bypass (i.e. catalyst warm-up enhancing operation) by opening the waste gate valve cannot be performed.
In particular, in a vehicle with a push-type starter switch that has widely been used in recent years, the engine is immediately started after ignition-on. Thus, enough time to perform the fully closed position learning is not provided before the start, which is disadvantageous from a perspective view of enhancing the catalyst warming-up.
In order to improve the exhaust gas processing performance immediately after the start, the opening control of the waste gate valve can desirably be executed immediately after the start of the engine.